Make "sys/queue.h" usable with C++

[Eric van Gyzen]

On 06/09/2015 05:54, Hans Petter Selasky wrote:
> On 06/08/15 11:25, Poul-Henning Kamp wrote:
>> It's really a decision if we want to spread dependence on typeof() 
>> to /usr/include isn't it ?
> 
> Hi Paul-Henning,
> 
> I'm aware about the typeof. Without typeof we would have to duplicate
> more of the queue macros, because in C++ you can make lists with both
> structs and classes. Else like some have suggested, we might drop the
> class/struct keyword, but that again will not work in external C
> sections in C++ files.

Did you test your claim in the last sentence?  I did.  Dropping the class/struct keyword in __cplusplus works fine under 'extern "C"'.  If you _did_ test and it failed, please show your code to clarify any miscommunication.

Dropping the class/struct keyword has the advantage that it does not change the API.  It has the disadvantage that types must be declared before they're used when #including <sys/queue.h> in C++ code.  The latter is simply good form.  There are three trivial declarations to fix in the src tree.  An exprun would find those in ports and give a feel for the incidence rate in non-ports code.  I would request this if anyone else is interested in this alternative.

	https://reviews.freebsd.org/D2767

Eric